Radio Broadcast (May 1928-Apr 1929)

RADIO BROADCAST Fig. 1 — Complete circuit diagram of the original National short-wave tuner. weird response effects with bumps of signal coming through where there should be none. If your band-pass filter does not perform properly it is suggested that you shunt a 500,000-ohm Frost resistor across the output, and, by pure cut and try, adjust the terminal conditions so that the desired action is obtained. When the action is correct the signal "snaps" in, stays for a while as the tuning dial of the receiver is turned, and then " snaps" out. Observe that the output circuit of the National timer has been altered a bit. This is to permit the 95 kc. output to enter the i.f. system, at the same time permitting the regeneration control to function. The numberings shows what has been done, as do the diagrams of Figs. 1 and 2. The r.f. choke has been eliminated, Cs has been moved, and the wiring has been changed slightly. These changes also improve the control when using the smallest tuner-coil. Rather than draw the complete schematic diagram it has been considered best to mark various posts of the adapter (Fig. 3), such as "A+," as often as necessary, even though only a single terminal is required in the receiver. Therefore, it should be understood that all binding posts with the same markings on either the set or the adapter are to be connected together. In the second i.f. stage a 222-type tube is shown. The gain obtained in this way is more than required, but if anyone desires more gain he is welcome to use a 222-type tube in the first i.f. socket as well — providing he can invent a way to match the high plate impedence to the lower impedance of the band-pass filter. This is strictly necessary to secure decent filter action, not to speak of decent gain. On the other hand, it is perfectly practical to use 20lA-type tubes in both of the i.f. positions. If this plan is followed the circuits will tend to oscillate and stabilization of some sort must be provided. The simplest thing is the old standby; return the grids to a potentiometer across the A supply and turn the knob to suit. The potentiometer may conveniently have a resistance of 400 ohms and the grid returns should be bypassed directly to their own filaments with O.lmfd. If anyone has available other i.f. transformers they may be used, provided the first contains a primary by-pass condenser. Usually it is of the "tuned" variety and has such a condenser. Frequencies materially above 100 kc. are not to be recommended because of the detuning required, while very low frequencies tend to cause difficulties from noise and damaged quality. Concerning A.C. Operation A RECEIVER akin to the one here described has been operated for some weeks with various portions of the circuit modified to permit the use of a.c. tubes. On the whole the performance has been satisfactory but previous experience with such matters teaches the writer to believe nothing about an a.c. job imtil it has been thoroughly time-tried. We must not stop without mention of television reception. If the transmission is being made with a 24-hoIe disc at 15 pictures per second, or a 48-hole disc at 1\ pictures a second, we have a "basic" frequency of modulation amounting to 360 cycles and a tolerably probable impulse frequency running up toward 9000 cycles. This means that the carrier plus both sidebands will be about 18,000 cycles wide, which is about twice as wide as the "pass" band of the Rusco filter. The set is, therefore, not good for the purpose unless a filter with a wider pass band is used, and even then it does not have much to recommend it since there are easier ways to attack the problem. For this sort of work it is recomended that an entirely different amplifier of the usual "television type" be used which can be done with the greatest ease as the tuner has not been incapacitated in any way. It should be noted that the tuner controls are at all times operated in the same manner whether it be us'ed with the "television" amplifier, the band-pass amplifier or the usual audio amplifier alone. Since mention has been made of satisfactory gain through the system it may be of interest to run hurriedly through the circuit with this in mind. The first 222-type tube, which is used as a "coupling tube," produces a gain of about 2, the autodyne detector produces a gain that is varied with adjustment and signal strength, the first i.f. tube (201a) produces a gain that is not up to the usual at such frequencies because of its peculiar plate load. The 222-type tube which follows the filter operates with a moderately good plate load and provides most of the gain in the i.f. system, which may be further improved by using a " tuned impedance" at this point, making sure that the condenser between this circuit and the next grid is of very high leakage resistance. The following 20lA-type tube, acting as second detector, produces the slight gain which is normal in that position and this is generally sufficient to cause the signal to overload either this tube or the 112A-type audio tube, although the latter is working under proper conditions. To take care of this condition a Frost highresistance rheostat has been mounted on the hitherto blank panel of the adapter and has been connected across the secondary of the first 95 kc. transformer. By a minor operation it has been modified so as to open at one end of the scale, thus permitting the removal of the shunt when it is not desired. If no very strong signals are encountered it is better to connect this control across the secondary of the second 95 kc. transformer since then it will have no effect on the detector regeneration. Another feature of the receiver described by the writer is that the use of the National steel cases and the various part shields results in a complete freedom from the bothersome hand capacity common to short-wave re Fig. 2 — Changes which must be made in the detector plate circuit of the tuner. ceivers. To complete this effect the panel of the right-hand (i.f. and a.f.) case was backed by a sheet of aluminum. List of Apparatus THE parts required for the construction of the double-detection short-wave receiver described in this article follow: One National screen-grid short-wave tuner; One blank panel, aluminum; One bakelite basepanel, 9" x 11"; One Rusco band-pass filter, 95-kc. ; Two Rusco i.f. transformers, 95-kc; Four tube sockets, ux-type, spring-construction; Three Carter resistors, 1-ohm, 2-ohm, and 15-ohm; Two Sangamo mica condensers, 0.001-mfd., and 0.0005mfd.; One Durham grid resistor, lj-megohm; One Twin-Coupler 222-type shield; Clips for grid leak, brass angles for connecting panel and base, machine screws, wire, binding posts, etc.